Apples go through an amazing transformation during their growth cycle. They start off as small green fruits and gradually turn into the brightly colored red apples that we love to eat. But why does this color change happen? The shift from green to red is the result of complex biological processes occurring inside the apple. In this article, we’ll look at what’s going on inside an apple to produce this spectacular color change.
Chlorophyll gives young apples their green color
When apples first emerge on the tree, they are small and green in color. This green color comes from chlorophyll, the main pigment found in plant leaves and stems. Chlorophyll’s job is to absorb sunlight and power photosynthesis, the process plants use to convert light energy into chemical energy.
Chlorophyll reflects green wavelengths of light, causing plant tissues with lots of chlorophyll to appear green to our eyes. Young apple fruits contain high levels of chlorophyll in their skin, giving them a characteristic green hue early in their development.
Why green chlorophyll masks red pigments
Apples also contain natural red pigments in their skin from the very beginning. These red pigments are called anthocyanins and belong to a class of pigment molecules called flavonoids. Apples produce several types of anthocyanins that give their skin and flesh a red, blue, or purple coloration.
So if apples have red pigments, why don’t we see them in young green apples? The reason is that the abundant green chlorophyll present in young apples masks the colors of the other pigments. Chlorophyll is very efficient at absorbing visible light for photosynthesis and overwhelms the colors of weaker pigments like anthocyanins. The red, blue, and purple hues provided by anthocyanins only become visible when chlorophyll levels decrease later on.
Why chlorophyll levels drop as apples ripen
As apples grow and mature, the amount of chlorophyll starts to decrease in their skin. With less chlorophyll present, the anthocyanin pigments that were there all along become visible, and the apple’s color shifts to red, purple, or orange-red. The drop in chlorophyll occurs for several reasons:
Lower light exposure
One factor is that as the apples grow larger, they receive less direct sunlight. The skin of larger fruits on the interior of the tree canopy is shaded by leaves and outer fruit layers. With less exposure to sunlight, they need to produce less chlorophyll.
Breakdown of chloroplasts
Chlorophyll is contained inside plant cell structures called chloroplasts. As fruits ripen, chloroplasts slowly break down, causing chlorophyll levels to decline.
Conversion into other pigments
Some of the chlorophyll molecules are converted into other pigments called non-fluorescent chlorophyll catabolites. Thesecatabolites don’t have a green color.
Unmasking of anthocyanins
With the drop in chlorophyll, the anthocyanin pigments already present in the apples become visible. This reveals the red, blue, and purple colors that were there all along.
Other color changes
In addition to unmasking anthocyanins, other pigment changes occur:
Carotenoid accumulation
Ripe apples accumulate carotenoids in their skins. Carotenoids are red, orange, and yellow pigments produced by the fruits. Different apple varieties contain different types of carotenoids.
Production of flavonols
Apples also synthesize flavonols late in ripening, which add extra yellow hues.
Why do apples change color?
In summary, apples experience major pigment alterations as they ripen:
- Chlorophyll levels decrease due to shading, chloroplast breakdown, conversion into other pigments, and unmasking by anthocyanins.
- Anthocyanin pigments already present become visible as chlorophyll declines.
- Carotenoid and flavonol pigments accumulate and add orange and yellow hues.
It is this complex shift in pigmentation that transforms green apples into the brightly colored fruits that charm our senses!
When do apples change from green to red?
Apples go through their color change at different times depending on the variety. The timing depends on when the ripening process gets triggered. Some general timelines:
- Early season apple varieties start changing color in mid-summer.
- Mid-season varieties like Gala or Honeycrisp turn red from late-summer through early fall.
- Late-season varieties change color in late fall leading up to harvest.
Within a single apple variety, fruits on the outer, sun-exposed parts of the tree will change earlier than shaded interior fruits. The fruits don’t change color evenly. Green patches may persist on some parts of the skin after other areas have turned red. Let’s look at the color development sequence in more detail.
Green to breaker stage
The first hint of color change is the appearance of trace amounts of red, orange, yellow, or purple pigments alongside the original green. Early in the season, examine apples closely to spot these subtle breaks in color on some area of skin. This is called the “breaker” stage and signals the beginning of ripening.
Partial red/green stage
Next, apples enter a partial red or orange stage where some of their skin turns red or orange but they retain large patches of green. This two-tone look persists for a while as pigments accumulate.
Full red/orange stage
Finally, the green areas disappear entirely, leaving apples fully red, rosy-red, or orange across their skin surface. This is considered the ripe stage for red and orange apple varieties.
Factors influencing apple color
Many factors affect the ability of an apple to produce its characteristic red color:
Apple variety
The variety’s genetic makeup determines the pigments it can produce. Some varieties lack red pigments entirely.
Sunlight exposure
Sunlight stimulates the production of red pigments. Shaded apples may fail to develop full color.
Temperature
Cool nights and sunny days promote red color formation. Hot temperatures hinder it.
Maturity and ripeness
Apples must mature fully before completing their color change. Picking apples too early means less developed red pigments.
Leaf:fruit ratio
Too many leaves competing with fruits can limit their color development. Pruning can help improve light exposure.
Crop load
Heavy crop loads prevent good coloration. Thinning fruits improves the leaf:fruit ratio.
Plant nutrition
Adequate nitrogen, phosphorus, potassium, and magnesium are needed for good color.
Water status
Drought stress reduces color development. Adequate irrigation is important.
How to manage apple color
Apple growers utilize various strategies to achieve optimum red coloration in their fruits:
- Select red-colored cultivars suited for their climate.
- Manage canopy density through pruning and training systems.
- Use reflective ground covers or foils to increase light interception.
- Position fruits so they receive direct sun exposure.
- Thin overcrowded fruits early in season.
- Maintain optimal soil fertility and plant nutrition.
- Irrigate to avoid drought stress.
- Harvest at mature stage when color is fully developed.
By mastering these horticultural practices, growers can maximize the prized red color that makes apples so appetizing.
Why proper color matters
Attaining full red color is not just an aesthetic goal – it has practical implications for apple quality:
- Consumer appeal – Shoppers judge ripeness and flavor based on an apple’s exterior color. Rich color is perceived as a sign of quality.
- Eating quality – Red skin color correlates with maturity, flavor, and texture. Greener apples are often starchy or acidic.
- Storage potential – Fully red apples store better and have a longer shelf life than less mature green apples.
- Nutrition – Red pigments are antioxidants that boost the nutritional value of apples.
In summary, excellent color is not only visually striking, but improves overall fruit quality. This explains why apple growers strive to optimize conditions for color development.
Why some apples stay green
While most apples turn red, certain varieties are meant to remain fully green when ripe. So why do some apples stay green? There are a few reasons:
Lack anthocyanins
Some green apple cultivars lack anthocyanin pigments in their skin and cannot turn red. Granny Smith and Pippin are examples.
Low light levels
Shaded inner canopy fruits may fail to deactivate chlorophyll and synthesize new pigments.
Insufficient maturity
Picking apples prematurely means they haven’t had time to complete the color change process.
Growing conditions
Excess nitrogen, high heat, drought stress or other factors can inhibit color development.
Genetic predisposition
Cultivars like Granny Smith remain green when fully ripe due to their genetics.
Conclusion
The transformation of green apples into red fruits is a fascinating case of changing plant pigmentation. As chlorophyll fades, existing red anthocyanins become visible. Additional yellow, orange and red carotenoids accumulate as part of the ripening process. The resulting surge in colorful plant pigments delights our eyes and taste buds! Through careful cultivation practices, apple growers can unlock the full color potential of their crop.
So the next time you crunch into a crisp red-skinned apple, appreciate the remarkable biology behind its sheer beauty and hidden nutrition!
